18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001
Lee Waves over Complex Topography during MAP
James D. Doyle, NRL, Monterey, CA; and R. B. Smith and G. S. Poulos
Poster PDF (3.1 MB)
The Hohe Tauern is a quasi-two-dimensional, east-west oriented range in the eastern Alps that includes the highest peak in Austria, Grossglockner at 3797 m. On 20 September 1999, during an extended period of deep-south foehn, a quasi-periodic wave train was observed in the lee of Grossglockner by a research aircraft equipped with a suite of instrumentation including downlooking Lidar and GPS dropsondes. The foehn was particularly intense on this day with a surface wind speed maximum of 50 m/s observed at Patscherkofel, near Innsbruck. It is noteworthy that in this case and a number of other situations during MAP, real-time predictions by the available mesoscale models forecasted vertically propagating large-amplitude waves rather than quasi-periodic waves in the lee, as was often observed.

Research aircraft observations, NRL's nonhydrostatic mesoscale modeling system, COAMPS, with high horizontal (grid increment of 1 km) and vertical resolution (60 levels), and the Smith linear model are used to provide new insight into the dynamics of lee waves over complex topography. Evidence of strong descent and "shooting flow" in the lee of Grossglockner that transitions into a series of well-defined periodic lee waves are apparent in both the observations and nonlinear model results. Upstream of the topography, blocking and stagnation occurs such that the flow above approximately 2.5 km is decoupled from the complex topography below, before plunging over the highest peak of the region, Grossglockner. Real-data simulations of the event replicate the lee wave characteristics including the horizontal wavelength of ~15 km, similar to that observed. Linear simulations also accurately represent the wavelength, but underestimate the amplitude of the waves by 50% or more underscoring the importance of nonlinearity. The simulations indicate that the waves are partially trapped and wave amplification and breakdown ensues in the lower-stratosphere apparently forced by reverse vertical shear above the jet stream. The upper-portion of a wave duct appears to be enhanced by latent heat release in the middle troposphere, which reduces the stability and Scorer parameter aloft. Surface viscous processes act to reduce the wave amplitude and prevent breaking. Downlooking Lidar observations indicate high-frequency structures were superimposed on the lee wave crests consistent with signatures of shear-induced turbulence. The simulations indicate that the effect of the lee-side boundary layer on the trapped waves may be complex. In some localized regions in the lee, a reduction in the Scorer parameter occurs near the surface suggesting that the boundary layer may reflect downward propagating waves upward prior to reaching the surface. Further downstream from the crest, the boundary layer becomes nearly stagnant leading to rapid absorption of downward propagating lee waves. The importance of three dimensionality will be addressed using the nonlinear model results and observations from a motorized glider that surveyed the lee wave train.

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